1. Field of the Invention
The invention relates generally to the field of well logging. More particularly, the invention relates to sensors for measuring acceleration due to earth""s gravity, and application of such measurements to evaluation of subsurface earth formations.
2. Description of Related Art
Measurements of the earth""s gravitational acceleration, and measurements of differences in the earth""s gravitational acceleration (gravity difference) between different depths in the earth, can be useful in determining the bulk density (or specific gravity) of various earth formations, among other applications. More particularly, measurements of gravity difference between two positions or depths may be used to determine whether oil, water or gas primarily fills pore spaces in the earth formations at various depths and geographic locations in the earth.
As a practical matter, measuring any physical property of earth formations beneath the surface of the earth is typically performed by a process called xe2x80x9cwell loggingxe2x80x9d, wherein instruments having various sensors therein are lowered into a wellbore drilled through the earth. The instruments may be lowered into the wellbore and retrieved therefrom at the end of an armored electrical cable, this process being known as xe2x80x9cwirelinexe2x80x9d well logging. Alternative conveyance techniques known in the art include lowering the instruments into the wellbore coupled to the end of a drill pipe, a production tubing or a coiled tubing. The drill pipe conveyance technique, in particular, is commonly referred to as xe2x80x9clogging while drillingxe2x80x9d when performed during the actual drilling of a wellbore. The well logging instruments, whether wireline or pipe conveyed, may include various devices to measure the earth""s gravitational acceleration.
One type of gravity and gravity difference measuring instrument is described in U.S. Pat. No. 5,892,151 issued to Niebauer et al. This instrument includes at least one, and preferably several longitudinally spaced apart gravity sensors enclosed in an instrument housing. The gravity sensors are fiber optic interferometry devices, which measure a velocity of a free falling mass by determining, with respect to time, interference fringe frequency of a light beam split between a first path having a length corresponding to the position of the free falling mass, and a second xe2x80x9creferencexe2x80x9d (fixed length) path. The fringe frequency is related to the velocity of the free falling mass, which can be correlated to earth""s gravity by precise measurement of the mass""s position and the time from the start of free fall. Measurement of gravity difference is performed by determining a difference in gravity measurements made between two of the individual gravity sensors positioned at vertically spaced apart locations.
One limitation to using gravity sensors such as those disclosed in the Niebauer et al. ""151 patent is that the accuracy of the gravity measurement may be insufficient for making gravity difference measurements in boreholes. Typically, to be useful in determining density of earth formations adjacent to a wellbore, it is desirable to have gravity difference measurements made to a vertical separation of about 1 meter or less. Gravity measurement sensors of the type disclosed in the Niebauer et al ""151 patent may have a noise level such that gravity difference between such small true vertical separations is below the needed accuracy for borehole applications. Another limitation to using the sensors such as disclosed in the Niebauer et al ""151 patent is that each individual sensor requires a separate optical path between a light source and a light detector. In reservoir monitoring applications, which may require many such sensors positioned at vertically spaced apart positions along a wellbore, and perhaps many such sensor xe2x80x9cstringsxe2x80x9d positioned within a plurality of such wellbores, signal transmission and processing may become very complicated. Furthermore, the implementation proposed in the Niebauer et al ""151 patent suggests that a laser source be positioned near the individual sensor. Because may wellbores have high temperatures within them, positioning a laser source in a well logging instrument, and more particularly a well logging instrument that may be permanently installed in a wellbore, may be impracticable.
What is needed is a gravity sensor and differential gravity sensor having accuracy suitable for reservoir monitoring, and having improved resistance to environmental effects.
One aspect of the invention is a gravity sensor which includes a first mass adapted to free fall when selectively released from an initial position. The first mass has optical elements on it which are adapted to change a length of an optical path through the sensor in response to movement of the mass. The sensor output is coupled to a beam splitter. One output of the beam splitter is coupled substantially optically directly to an interferometer. Another output of the splitter is coupled to the interferometer through an optical delay line. A frequency of an interference pattern generated in the interferometer is directly related to gravity at the location of the free falling mass.
Another aspect of the invention is a gravity difference sensor. A second mass, adapted to free fall when selectively released and having similar optical elements on it as the first mass, is optically coupled in series with the first mass. The second mass is adapted to change the optical path length in opposed sign or sense to the change caused by the first mass when it is dropped. The second mass is selectively dropped to have at least partially time coincident movement of the first and second masses. An interference pattern generated in the interferometer in this case has a frequency related to gravity difference between the first mass and the second mass.
A method for measuring gravity according to another aspect of the invention includes illuminating optical elements on a first mass that is adapted to free fall when released from an initial position. The optical elements are adapted to change a length of an optical path in response to movement of the mass. The mass is dropped at a selected time. Light traveling away from the optical elements is split. One output of the splitting is conducted substantially directly to one input of an interferometer. Another output of the splitting is conducted to the interferometer through a selected time delay. A frequency of an interference pattern in the interferometer is then determined. The frequency of the interference pattern is related to gravity at the first mass.
A method for measuring gravity difference according to another aspect of the invention includes, prior to the splitting, directing the light leaving the optical elements on the first mass to optical elements on a second mass. The second mass is also adapted to free fall when selectively released from an initial position. The optical elements on the second mass are adapted to change the length of the optical path in response to movement of the second mass in opposite sign or sense to the length change caused in response to movement of the first mass. The second mass is selectively dropped. The selective dropping of the second mass is timed to enable at least partially time coincident movement of the first mass and the second mass. A frequency of an interference pattern in the interferometer is determined during the time coincident movement of the first and second masses. The time coincident movement interference pattern frequency is related to gravity difference between the first and second masses.
Other aspects and advantages of the invention will become apparent from the following description and the drawings.